Problem: $ D = \left[\begin{array}{r}2 \\ 5 \\ 4\end{array}\right]$ $ B = \left[\begin{array}{rr}-1 & 5\end{array}\right]$ What is $ D B$ ?
Solution: Because $ D$ has dimensions $(3\times1)$ and $ B$ has dimensions $(1\times2)$ , the answer matrix will have dimensions $(3\times2)$ $ D B = \left[\begin{array}{r}{2} \\ {5} \\ \color{gray}{4}\end{array}\right] \left[\begin{array}{rr}{-1} & \color{#DF0030}{5}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ D$ , with the corresponding elements in column $j$ of the second matrix, $ B$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ D$ with the first element in ${\text{column }1}$ of $ B$ , then multiply the second element in ${\text{row }1}$ of $ D$ with the second element in ${\text{column }1}$ of $ B$ , and so on. Add the products together. $ \left[\begin{array}{rr}{2}\cdot{-1} & ? \\ ? & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ D$ with the corresponding elements in ${\text{column }1}$ of $ B$ and add the products together. $ \left[\begin{array}{rr}{2}\cdot{-1} & ? \\ {5}\cdot{-1} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ D$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ B$ and add the products together. $ \left[\begin{array}{rr}{2}\cdot{-1} & {2}\cdot\color{#DF0030}{5} \\ {5}\cdot{-1} & ? \\ ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{2}\cdot{-1} & {2}\cdot\color{#DF0030}{5} \\ {5}\cdot{-1} & {5}\cdot\color{#DF0030}{5} \\ \color{gray}{4}\cdot{-1} & \color{gray}{4}\cdot\color{#DF0030}{5}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}-2 & 10 \\ -5 & 25 \\ -4 & 20\end{array}\right] $